The circuit diagram on the X10 tutorial is incorrect. It does not work at all.

http://www.arduino.cc/en/Tutorial/X10

This has been discussed before:

http://arduino.cc/forum/index.php/topic,17333.0.html

The problems are as follows:

1) The Zero Crossing and Data lines are swapped.2) The jack orientation is confusing and should be marked with pin numbers.

Here is a suggested replacement:http://imagepaste.nullnetwork.net/viewimage.php?id=1689

Also, the only X10 command example shown in the tutorial is the "ALL_LIGHTS_ON" command, which not all modules respond to. Addressing a single module is not particularly intuitive. An example of this could be added as well: